You don't always design bigger systems from scratch as greenfield applications, most of the time you are delivering smaller features in incremental ways or fixing bugs. Even for greenfield projects, you can plan with the LLM and come up with a design you like at the high level but that still does not spare you from knowing (or not) how lower levels work, which is OP's original complaint.

In essence you basically say that you don't read or review the code produced in detail, you just know the high level design of how the system works.